From Coastal Wiki
Definition of Pelagic:
Designating, relating to, or inhabiting that region of the sea which consists of open water of any depth, away from or independent of both the shore and the sea floor (and so contrasted with the littoral and benthic regions).
This is the common definition for Pelagic, other definitions can be discussed in the article
The Wikipedia provides a detailed description of the zone see (http://en.wikipedia.org/wiki/Pelagic)